The overarching goal of this work is to understand the process of sexual differentiation of the brain. Currently, the hormonal control of cell death is the best-established mechanism for creating sex differences in the nervous system. Nonetheless, very little is known about how hormones such as testosterone regulate neuronal cell death. This project will examine the mechanisms underlying hormonally controlled cell death in three well-studied model systems of the brain and spinal cord: the bed nucleus of the stria terminalis (BNST), the anteroventral periventricular nucleus (AVPV) of the hypothalamus, and the spinal nucleus of the bulbocavernosus (SNB). Mice will be used throughout, to take advantage of the power of genetically manipulated strains. Testosterone decreases cell death in the BNST and SNB, and increases it in AVPV. To specify the steps between production of testosterone and the decision of a cell to live or die, and to gain insight into how testosterone exerts opposite effects on cell survival in the different nuclei, the hormone metabolites and receptors that mediate hormonal effects will be identified in Aim 1. It will be determined whether androgenic or estrogenic metabolites of testosterone control sexual differentiation in the three model systems, and receptor subtype-specific agonists will be used to zero in on the specific receptors involved. The pro-apoptotic protein, Bax is required for cell death in the BNSTp, AVPV, and SNB. In Aim 2 the subcellular localization of Bax, and the expression of proteins that interact with Bax will be examined in control and testosterone-treated animals. Because testosterone pushes cell death in opposite directions in the BNST and AVPV, it is predicted that hormone treatments will differentially affect death-regulatory proteins in these neural areas. Finally, Aim 3 will test the hypothesis that chromatin remodeling is required for sexual differentiation of the brain. This hypothesis will be tested by examining the effect on sexual differentiation of disrupting histone deacetylation during a critical perinatal period and by identifying hormone-dependent changes in histones associated with the cell death genes shown to be important in Aim 2. Together, these studies will determine at a mechanistic level how hormones control sexual differentiation of the nervous system. This work is relevant to understanding sex differences in susceptibility to human neurodevelopmental disorders, and psychiatric and neurodegenerative diseases in adulthood.